The invention relates to dendrimeric fullerene derivatives, that is fullerene derivatives which are substituted by one or more dendrons. Fullerenes are carbon compounds of the formula C2(10+m), where m is a natural number. They contain twelve five-membered and any desired number, i.e. m, but at least two, six-membered rings of carbon atoms.
The outwardly vaulted surface of fullerenes and the alignment of the xcfx80 electrons produced thereby cause a great reactivity to free radicals /1/ (The literature references are listed at the end of the description). Thus, it is known for C60 Buckminsterfullerene that it very easily adds radicals. The absorption of up to 34 methyl radicals is described and the name xe2x80x9cradical spongexe2x80x9d has been proposed therefor /1/.
It is known that hydroxyl and hydroperoxide radicals in biological systems preferably attack polyunsaturated fatty acids. This attack brings about crosslinking and polymerization of the fatty acid structures. Free radicals thus also bring about damage to biomembranes, in particular the membranes of nerve cells, which are distinguished by a particularly high proportion of unsaturated fatty acids in their lipids. As a result of the change in the lipid structure of such biomembranes, a change in the membrane permeabilities, a change in their transport functions, a change in their barrier mechanisms and changes in their receptor activities can occur and this can lead to the death of the nerve cells. Overstimulation of glutamate receptors likewise led to the formation of free oxygen radicals and nitric oxide radicals /2-6/ and as a result to damage to nerve cells and to the occurrence of specific syndromes /7-10/.
Radical scavengers provide for the removal of harmful free radicals, act as antioxidants and suppress the damage to the nerve cells and the loss of function of neuromembranes. Therefore, especially fullerenes with their excellent radical-scavenging properties should act as neuroprotectants and should be able to be employed for the protection of biological membranes against oxidative changes. C60 fullerene, however, is only soluble in very few organic solvents, such as benzene and toluene, and thus cannot also be administered to biological membranes. The addition of suitable functional groups on C60, however, should improve the water solubility and thus also make possible use as antioxidants in biological systems. The first positive results were achieved using polyhydroxylated C60/11/, and trisubstituted synthetic malonic acid derivatives C60[C(COOH)2]3, which had both in vitro and in vivo neuroprotective activities, afforded better results /12/. The preparation of the two isomers having C3 or D3 symmetry described in /12/ was carried out by the method of Hirsch /13/ by triple cyclopropanylation of C60 with diethyl malonate, hydrolysis and subsequent chromatographic purification. The two test compounds exhibited strong affinity for free radicals, as it was possible to show by EPR spectroscopy, and inhibited the excitotoxic death of cell cultures of cortical neurons which were induced by N-methyl-D-aspartate (NMDA) exposure, by xcex1-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) exposure or by oxygen/glucose withdrawal. The C3-symmetrical derivative moreover reduced apoptotic neural death which was induced either by serum withdrawal or exposure of Axcex21-42 protein. In all cases, the C3 regioisomer exhibited greater efficacy than the D3 isomer, which can be attributed to the greater polarity and thus improved ability of lipid membrane penetration /12/.
Relative to the polyhydroxy-substituted fullerenes, the malonic acid derivatives exhibited better activity as radical scavengers as it [lacuna] with the lower degree of addition and resulting from this an improved provision of a relatively large reactive surface with, at the same time, accompanying improved water solubility.
Furthermore, fullerenes are known from /17/ which are linked to poly(benzyl ether) dendrons. These fullerene derivatives, however, are insoluble in water. In /19/, dendrimeric fullerenes are likewise disclosed which are insoluble in water.
In relation to this prior art, the invention was based on the object of further improving the water solubility of fullerenes or fullerene derivatives without at the same time, as in the cases known to date, making smaller or screening off the reactive C60 surface and thus reducing the radical-scavenging properties.
This object is achieved according to the invention by dendrimeric fullerene derivatives in which the fullerene is linked to at least one dendron, wherein the or each dendron has at least one protic group which imparts water solubility.
Another object of the invention further relates to an agent comprising the inventive fullerene for thereapeutic purposes. The invention additionally relates to an agent comprising the inventive fullerene as neuroprotectant. The inventive fullerene derivates can be used in the production of pharmaceuticals for the treatment of diseases in which free radicals have a harmful effect on the organism to be treated.
Suitable protic groups are the hydrophilic groups known to the person skilled in the art, preferably xe2x80x94OH, xe2x80x94COOH, xe2x80x94NH2, xe2x80x94SO3H, xe2x80x94PO3H, NR4+, xe2x80x94NHOH, or xe2x80x94SO2NH2, where the four radicals R independently of one another can in each case identically or differently be xe2x80x94H, alkyl or aryl. xe2x80x94OH, xe2x80x94COOH and xe2x80x94NH2 are particularly preferred. The group xe2x80x94COH can also be employed.
The water solubility of the fullerene derivatives according to the invention can be greater than 1 mg/ml at 25xc2x0 C. and pH 7, preferably greater than 5 mg/ml, particularly preferably greater than 10 g/l and very particularly preferably greater than 20 mg/ml. At 25xc2x0 C. and pH 10 it can be greater than 10 mg/ml, preferably greater than 50 mg/ml, particularly preferably greater than 100 g/l and very particularly preferably greater than 200 mg/ml.
A dendron within the meaning of the invention is an addendum of the C60 nucleus unit which has a branching at the end as a structural unit. Again, in each case the same or another subunit, at whose other end a branching is likewise located in turn, can be linked to this. Depending on the degree of branching, the number of the nearest branchings and thus also the number of functional (and water-soluble) end groups is thus doubled, tripled or multiplied. Each radially symmetrical xe2x80x9cshellxe2x80x9d formed by repeated linkage of dendrons is designated as the next generation. In order to make available more reactive surface for radicals, it appears sensible to keep the dendrons necessary for the water solubility at as far a distance as possible from the C60 core in order to prevent its xe2x80x9ccoveringxe2x80x9d. This is possible by binding of the dendrons of the first generation via a so-called xe2x80x9cspacerxe2x80x9d, i.e. a carbon chain of length C1 up to about C100, preferably C2-C10, which serves the 1st generation as a xe2x80x9cseparatorxe2x80x9d.
Suitable branchings are especially trivalent or polyvalent elements such as, for example, N-, C-, P-, Si, or polyvalent molecular segments such as aryl-, heteroaryl. The degree of branching is preferably between two and three and the number of generations can be between 1 and 10 inclusive.
Preferred solutions to the object according to the invention follow from the subclaims. Individual features or a number of the features disclosed in the subclaims can also in each case represent solutions to the underlying object per se or in combination and the individual features can also be combined in any desired manner within the claim categories.
In relation to the examples mentioned further above, monoadducts of fullerenes should be an improvement if their water solubility is adequately high. The lower the degree of addition of the fullerenes, the greater their free reactive surface will be, which appears responsible for the radical-scavenging properties. Such monoadducts, however, should have a lower water solubility in relation to polyadducts, as the number of hydrophilic groups decreases correspondingly. This conflict is solved by the fullerenes according to the invention as they in turn arbitrarily increase in water solubility owing to the branchings of the higher generations lying further outside.
A preferred compound is shown in formula 1 (Scheme 1). What are concerned here are dendrimer-branched derivatives of C60-fullerenes. Monosubstituted dendrimers of this type offer the advantage that they retain their excellent water solubility independently of the degree of addition owing to the choice of suitable functional end groups. The number of the functions of the outermost dendrimer shell contributing to the solubility is additionally specified by the degree of branching and by the number of dendron generations (cf. section above). Branching elements of the individual molecular esters can inter alia preferably be xcex3-amino-triscarboxylic acids of the type shown in formula 1, xcex1-aminodicarboxylic acids, xcex1-xcfx89-diaminocarboxylic acids, hydroxycarboxylic acids, tartaric acid derivatives, polyphenols, carbohydrate components of the pentose and hexose type, glycerol derivatives and also generally polyfunctional compounds. The consecutive linkages of the individual dendrimer branches or of the individual generations then corresponds to acid amide compounds, peptide compounds, ester bonds, depsipeptide-like bonds according to the nature of the variable acid amide or ester linkage of amino acids and hydroxy acids, glyceride-like bonds and polyesters or polyethers, polyphenyl ethers and glycosides. An adverse effect of the free fullerene surface due to screening is only expected in the case of dendrimer shells of the third generation, screening on account of easy penetrability of small free radicals, nevertheless does not necessarily have to lead to a decrease in the antioxidant activity. Owing to the choice of suitable lengths of two up to six carbon atoms in the spacer chains, the availability of the fullerene surface can additionally be varied or guaranteed. The use of long fatty acid chains in the individual branches can additionally bring about an affinity for the fatty acid chains of the lipids within the membrane bilayer and thus improved intercalation in nerve membranes and improved radical-scavenging properties. 
The preparation of the target compounds is carried out by convergent synthesis, i.e. the corresponding dendrimer branch is synthesized separately up to the first, second, third or nth generation and only finally bonded to C60 fullerene by means of a suitable linkage reaction, such as, for example, a cyclopropanylation, Diels-Alder reaction, [3+2] cycloaddition and the like. For the synthesis of the fullerene dendrimer of the first generation (cf. preparation example 8) according to Scheme 2, the O-benzyl-protected xcex3-hydroxybutyric acid 2 is condensed with the carboxy-protected xcex3-aminotriscarboxylic acid [Behera amine, di-t-butyl 4-amino4-(2-t-butyloxycarbonylethyl)heptanedioate 3 (/14/, /15/, /18/) by reaction with condensing agents and activators such as, for example, dicyclohexyl-carbodiimide [DCD], 1-hydroxybenzotriazole [1-OH-BT], but also other carbodiimides, N-hydroxy compounds, and the like. The acid amide [di-t-butyl 4-(4-benzyloxybutyrylamino)4-(2-t-butyloxycarbonylethyl)heptane-dioate 4] resulting therefrom can be deprotected by hydrogenolysis or specific, gentle hydrolysis to give the free hydroxy acid amide [di-t-butyl 4-(4-hydroxybutyrylamino)-4-(2-t-butyloxycarbonylethyl)heptanedioate 5] without affecting the protective groups of the carboxyl functions in the course of this. The following reaction of 5 with malonyl dichloride and pyridine or other nitrogen bases leads to the dendron 6 of the first generation [nomenclature for cascade polymers: 6-cascade:methane[2]:(2-aza-7-oxa-3,8-dioxooctylidyne):tert-butyl propanoate], which is a dialkyl malonate ester with amide-like dendron linkage and a C4-long spacer chain between the branching unit and the malonate residue. Correspondingly, by choice of other benzyl-protected xcfx89-hydroxycarboxylic acids 2 such as, for example, 3-hydroxypropionic acid, 5-hydroxyvaleric acid or 6-hydroxyhexanoic acid, the spacer chain can also be shortened or lengthened in any desired manner. According to a process described by us /16/, C60 can then be cyclopropanylated in the presence of CBr4 and DBU/methylene chloride using synthon 6. The tert-butyl protective groups ofthe resulting 
dendrimer fullerene [6-cascade:methano-C60-fullerene[2]:(2-aza-7-oxa-3,8-dioxooctylidyne):tert-butyl propanoate 7] can be easily removed by hydrolysis, preferably using formic acid, so that the polycarboxy-functionalized dendrimer fullerene 8 which is water-soluble and can be employed as a neuroprotectant is formed. [6-Cascade:methano-C60-fullerene[2]:(2-aza-7-oxa-3,8-dioxooctylidyne):propanoic acid.
The dendrimer fullerene 1 of second generation [18-cascade:methano-C60-fullerene[2]:(2-aza-7-oxa-3,8-dioxooctylidyne):(2-aza-3-oxopentylidyne):propanoic acid] (cf. preparation example 1) is prepared (Scheme 3) analogously to the synthesis of the derivative of first generation 8 shown in Scheme 2. As in the first case, the tert-butyl-protected xcex3-aminotriscarboxylic acid 3 is reacted with the appropriate nitrotriscarboxylic acid 9, whose tert-butyl ester is the synthesis precursor of 3 (15, 16), using DCC in the presence of 1HO-BT and subsequent reduction with hydrogen to the dendron 10 of second generation [9-cascade:aminomethane[3]:(2-aza-3-oxopentylidyne):tert-butyl propanoate]. The analogous reactions as for the preparation of 8 lead to linkage with the spacer 2, formation of the corresponding malonic acid diester and cyclopropanylation to give a carboxyl-protected dendrimer fullerene derivative. The analogous removal of the protective groups leads to the dendrimer fullerene 1 of the second generation. The latter is already very readily soluble in water (pH  greater than 7), MeOH and other polar solvents. In water, a solubility of about 34 mg/ml at 25xc2x0 C. and pH 7 was measured for the compound mentioned. The solubility at pH 10 was about 250 mg/ml. 
A third condensation reaction between the amino derivative 10 and the nitrotriscarboxylic acid 9 leads to a dendron 11 of the third generation [27-cascade:aminomethane[3]:(2-aza-3-oxopentylidyne)2:tert-butyl propanoate], which in turn can be converted by spacer lengthening into the cyclopropanylation reagent 12 of the third generation. Addition to fullerene C60 under the conditions already described yields a dendrimer 13 of the third generation having a C60 as core unit. Deprotection of the tert-butyl groups in turn yields a water-soluble dendrimer fullerene [54-cascade:methanofullerene[2]:(2-aza-7-oxa-3,8-dioxooctylidyne):(2-aza-3-oxopentylidyne)2:propanoic acid 14] (Scheme 4) of the third generation, which should be distinguished by radical-scavenging properties and should have membrane-protective and neuroprotective properties. 